This invention relates to automatically controlled electric circuit interrupting devices and more particularly to such devices that interrupt electric power supplied to a kiln in response to more than one parallel input stimulus.
In the operation of kilns it has long been the practice to utilize pyrometric cones to determine when the process of firing a work piece in the kiln has been completed. In a representative case, the kiln operator will observe such a cone in the kiln firing environment for physical changes. It is a characteristic of the material of such cones that the cone becomes softened after exposure to elevated temperature for a sufficient length of time. The cone will suffer a physical distortion thus altering the kiln operator that the process is complete.
Advantage was taken of this physical distortion in U.S. Pat. No. 3,287,530, issued to W. P. Dawson Nov. 22, 1966. A pyrometric cone, utilized as support for an actuator rod, becomes softened by time exposure to an elevated temperature in the kiln. Upon such softening, the pyrometric cone no longer provides support for the actuating rod thus allowing release of a contact-opening mechanism so as to open the electric circuit supplying power to the heating elements of the kiln.
It is desirable to provide accompanying control means, operable in response to stimuli other than the physical distortion of the pyrometric cone, in order to invest the kiln operator with additional control over the treatment of a kiln-fired workpiece. It is further desirable to provide an interlock mechanism by means of which power to the heating elements of the kiln may be interrupted by opening the lid of the kiln.